1. Field of the Invention
The present invention refers to ignition coils.
2. Prior Art
The engines of portable motorized saws or chainsaws are today very small in their outer dimensions, so that the ignition apparatus comprising the flywheel magneto must of necessity have very small dimensions. However, it is very difficult to accommodate an ignition coil inside a flywheel casing, and it must therefore be placed somewhere else. The coils in known apparatus, which are arranged inside the flywheel magneto casing, are automatically cooled by the rotation of the flywheel. An exteriorly mounted coil is generally fitted on or close to the engine block. Undesirable engine heat is thus transmitted to the coil. If the coil is placed on the chainsaw frame, the cables to it will unavoidably be relatively long, and as a result of the high frequency vibrations at very high revolutions, vibrations interrupting electrical contact easily arise, especially in the high-tension cable from the transformer, causing uneven running of the engine or complete operational breakdown. Neither is it desirable to arrange long high-tension cables in, or in the vicinity of, the saw frame, with the accompanying risk of spark-over. As is well-known, a chainsaw most often operates under severe conditions with respect to moisture and vibrations.
There are already many proposals for building a spark plug and an ignition coil together. The disadvantages of these assemblies is that the coils are subjected to severe heating as well as heavy vibrations, and the requirement for winding strength in such arrangements is therefore extremely difficult to meet. There is otherwise a great deal to gain by building the spark plug together with the ignition coil. High-tension cables can be avoided altogether, and it is easy to achieve great security against the entry of moisture when the devices are properly encased. A disadvantage is, however, that special spark plugs including the built-on ignition coil must be manufactured.
The present invention relates to a simple and well-adapted solution of the problems mentioned above, in which there are all the advantages of a unified assembly of the plug and coil, and none of the disadvantages mentioned hereinbefore.